Diamond turning has been traditionally used for the production of monolithic lens arrays, which are used in a variety of applications where lens-to-lens registration is of critical importance. The diamond turning process requires that the substrate, where the monolithic lens arrays are to be produced, is shifted to a new position before the next lens is machined, thereby requiring the operator to shift and align the substrate, then rebalance the work spindle for each lens position. As a result, the use of on axis diamond turning is commonly considered to be unsuitable for the production of large monolithic lens arrays due to the increase in production time required and difficulty in achieving lens-to-lens registration accuracy. In order to reduce the production time required, freeform techniques can be used for the production of monolithic lens arrays.
Gregg et. al. in “Comparison of Freeform Manufacturing Techniques in the Production of Monolithic Lens Arrays” provides a comparison of different freeform manufacturing processes that can be used in the production of monolithic lens arrays, which are discussed in more details below. Freeform diamond machining allows lens arrays to be produced in a single setup. Since there are no intermediate shifts of the substrate, the lens-to-lens registration is inherent to the program and machine accuracy. These freeform machining technologies are generally known as Slow Tool Servo (STS), Fast Tool Servo (FTS) and Diamond Micro Milling (DMM). However, the freeform machining technologies listed above have limitations with regards to the surface form accuracy, surface geometry, and production time required.
Diamond Micro-Milling is an effective method used in precision machining of small features on the surface of a work-piece using a spherical diamond tool. Micro-milling allows versatile freeform machining and has the advantage that the work-piece is not rotating. However the accuracy of the micro-milling process can be affected by certain factors relating to the spindle, machine kinematics errors, the misalignment of the tool shaft on the spindle axis, the waviness of the diamond ball end of the cutting tool and cutting tool vibrations caused by the interrupted cutting conditions. Even if the systematic errors caused by the factors mentioned previously are corrected using a correction cycle, the surface quality of a micro-milled surface is generally lower than the surface quality of a diamond turned surface. Another drawback of the diamond micro-milling process is the significantly longer times required for machining small features on the surface of the work-piece. Therefore, the machining on the surface of the work-piece of an array of a thousand lenslets can take up to several days of continuous machining.
Slow Slide Servo, also called Slow Tool Servo, and Fast Tool Servo use multiple axis synchronization, whereby the work-spindle is used as an axis, on a lathe to machine freeform surfaces. However, these methods do not allow the machining of steep slopes because of the limited clearance of diamond tools, are limited in speed by the machine kinematics and generate important surface error due to discontinuities in the toolpath.
Another aspect in the precision machining of small features on an object, such as lens arrays on a monolithic work-piece, is the ability of precisely positioning the object with respect to a tool, such as a cutting tool. Work-piece indexing is a known method used for locating a specific position of the work-piece with respect to a tool, such a cutting tool, for performing precision machining operations. Automatic positioning and indexing of a tool or work-piece is known from the prior art for many operations as for milling, drilling, laser machining, inspection, metrology, etc. These systems generally integrate guideways, actuators e.g. motor and position measurement instruments while the control system can be integrated or external. However, the automatic indexing or positioning devices of the prior art do not integrate balancing means for balancing the work-piece on a rotating spindle each time a new position is reached. As a result, the positioning and indexing solutions of the prior art are not suitable to be mounted onto a rotating precision work-spindle that needs to be accurately balanced on the rotating axis of the work-spindle. The automatic positioning and indexing solutions of the prior art are further not suitable to maintain precisely and firmly the tool or work-piece in its off-axis positions while the spindle is rotating at relatively high speeds, typically between 300 and 2000 rotations per minute (RPM), for example 300 to 1000 RPM.